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For people with hypothyroidism, the most common medication is levothyroxine, a synthetic form of thyroid hormone. It's an effective hypothyroidism treatment, but it's also a very tricky one — it can take weeks or even months to figure out the daily dosage that works best for you. Even then, if you don't follow your doctor’s instructions closely and take your medication properly, you could throw your hormone levels out of whack.

Finding the Right Dose of Thyroid Medication

To determine your levothyroxine dosage, your doctor will draw your blood to test for thyroid-stimulating hormone (TSH). This test measures how much thyroid hormone your body is asking your thyroid gland to make. The higher your TSH level, the more starved your body is for thyroid hormone — a sign that your doctor needs to increase your dose of the synthetic replacement hormone.

Unless your TSH levels are unusually high, your doctor typically will start you out at a low dose of about 50 micrograms a day. Then you'll probably have a TSH test every three to four weeks, and the readings from those tests will be used to determine your precise hypothyroidism treatment. Based on the test results, your doctor may increase or decrease the dose until your thyroid hormone levels stabilize.

"You’ll likely get to a steady level in around a month, and then four to six weeks is the rule of thumb to re-check your levels again," says Stephanie Lee, MD, PhD associate chief of endocrinology, nutrition, and diabetes at Boston Medical Center and an associate professor at the Boston University School of Medicine.

If you find that your levels are not stabilizing, you and your doctor will need to discuss alternatives. Your doctor might prescribe another type of levothyroxine or see if you respond better to natural thyroid hormone or a combination of different thyroid hormones.

Following Thyroid Treatment Instructions

It's very important to follow your doctor's instructions when taking medication as part of your hypothyroidism treatment. If you don't, your body may not be able to fully absorb the levothyroxine and your hormone levels could fluctuate.

"This hormone is potent in very small amounts, so any little change in your routine can change your hormone levels," says Laura Pizzi, PharmD, a professor at the Jefferson School of Pharmacy at Thomas Jefferson University in Philadelphia.

Small changes that can influence the effect of thyroid medication on your body include:

Changing the time of day you take your dose

Taking the medication on an empty stomach, then switching to taking it with food

Switching brands of levothyroxine

For the best levothyroxine absorption, Hossein Gharib, MD, a professor of endocrinology at the Mayo Clinic in Rochester, Minn., recommends that it be taken in a fasting state, just after you wake up. "If you can't take it in a fasting state, they you should take it in the evening, two to three hours after your evening meal,” Dr. Gharib says.

Watching for Interactions With Levothyroxine

Other drugs and some dietary supplements can interact with levothyroxine. For example, iron and calcium supplements can interfere with the absorption of levothyroxine. The iron or calcium can bind with the synthetic hormone, preventing your body from using it. "You're basically snatching away some of the levothyroxine that otherwise would’ve been absorbed into the body," Dr. Pizzi says.

The hormone estrogen also can interfere with the body's ability to use levothyroxine. That means women taking birth control pills may need an increased dose of thyroid medication, Pizzi says.

Other medications that can interfere with the absorption of levothyroxine include:

Monitoring Potential Side Effects of Thyroid Medication

Not taking levothyroxine the right way or experiencing drug interactions can make your thyroid hormone levels swing high or low and cause side effects. Elevated hormone levels can make you feel shaky, jittery, and jumpy, Pizzi says. You might notice your heart racing or have trouble sleeping at night. You might even lose some weight.

Hormone levels that are too low can cause fatigue and sluggishness. You might experience a slow heart rate, lose some of your hair, feel depressed, or gain weight.

If you experience any of these problems, call your doctor. "Your doctor will likely ask you to come in and get your blood checked," Pizzi says. Based on the results, he or she will probably change your thyroid medication dosage to address the side effect.

Overall, if you establish and maintain a regular routine with your hypothyroidism treatment, the condition should have little effect on your long-term health and well-being, Pizzi says.